A porthole door for a front-loading laundry appliance is described in DE 100 53 413 A1. The porthole door includes a door glass which is curved toward the inside of the appliance and has a frame composed of a visible cover frame and a holding frame, which is concealed when in a closed position. The door glass is held by its edge between the cover frame and the holding frame. In order to achieve optimal centering of the door glass, the inner diameter of the holding frame is sized to match the contact region of the door glass very accurately, i.e., with a small tolerance. An additional observation window on the outside of the porthole door covers the door glass curvature, which is directed to the inside of the appliance. The edge of the cover is located in a visible area on the outside. A snug fit is achieved by a clamped connection, which is provided by an axially outwardly facing edge lip of the cover frame pressing on the edge of the observation window. In this holding arrangement, the observation window and the cover frame must have a high degree of dimensional accuracy, because otherwise it is not possible to ensure a firm and permanent clamped connection. Slight deviations in dimensional accuracy may result in the inability to assemble the components, or in the necessity of ensuring a centered orientation when the gaps are too large. Moreover, if the door glass and/or the viewing pane is/are not centered, there is a risk of the door not bearing evenly against the door seal and not closing completely tight.
German Patent DE 195 15 040 C2 also describes a porthole door having an observation window. In that patent, the observation window is at least partially clamped against a resilient ring under a projection of the edge of the cover frame. In this arrangement, there is a visible gap between the edge of the observation window and the inner edge of the cover frame. Moreover, in order that the visible gap be very small and not have any visible irregularities, the observation window and the cover frame must have a dimensional accuracy within a small tolerance range.
German patent DE 101 37 918 C1 describes a door for a front-loading laundry appliance suitable for integration into a row of kitchen units. In that patent, the door glass is mounted on a closed front panel. For ease of assembly, a centering ring having a raised peripheral edge is attached to the front panel. The inner diameter of the door glass is put over the edge, thereby moving the door glass to the intended position. The centering ring is elastic in some portions, making it possible to compensate for differences in height of the peripheral edge of the door glass.